Many air conditioning zone control systems include a single sensor that monitors the temperature of the ambient air or the temperature of the air returning from the zone whose temperature is being controlled. This sensor provides feedback to an air-conditioning system controller in order for the controller to adjust various air-conditioning components such as supply air fans, coil coolant fluid proportional valves (cooling or heating mode), and electric heaters, if any, to attempt to maintain a temperature setpoint in the zone.
A controller using a control algorithm that only references a zone's ambient or return air temperature against a user entered setpoint can yield large temperature fluctuations in the zone because, for example, the supply air is of a much lower temperature than the zone's temperature. When this supply air is delivered to the zone, it causes a large temperature drop below the setpoint and the controller must affect an immediate adjustment in the opposite direction to provide warmer air to the zone.
This cycling is undesirable because it causes the controller to frequently adjust the system components in an effort to achieve the setpoint in the zone, and as a result, it merely increases equipment wear and causes periodic temperature fluctuations above and below the setpoint in the zone.
Certain algorithms exist to minimize the aforementioned cycling using only the ambient temperature in the zone or the return air sensor, but without additional sensors, they are not capable of providing for optimization of the temperature fluctuations in the coolant fluid, detecting air-conditioning system mechanical component failures, providing supply end equipment overheating alerts to a system user or building management system, or providing smart temperature controls for an air conditioned zone.